Normal: We saw in the Pokemon game trailer, Sylveon used four moves; some were hard to identify but but two are obvioulsy Trump Card and Swift, both Normal type moves. However, all the Eevee evolutions can learn both of those moves so in one way there's no proof yet there is. Plus Sylveon does reminds everyone of the cute Hoenn normal type kite Skitty due to the similar color contrast. But still, there is a less of a chance that this could be a Normal type because Eevee already is one.

Psychic: Originally, Sylveon wouldn't be this type because there already is a Psychic type: Espeon. However, if Espeon didn't exist then this would be a possible type due to color contrast.

Bug: Some people believe Sylveon to be a Bug type not only due to the butterfly like bows on it's ear and neck, but also because two of it's other language names who I can't remember at the moment resembled close to butterfly or insect of some sort.

Fighting: Fighting can be a slight possibility due to the contrast and scarf patterns, but no one can be too sure from lack of evidence.

Flying: One of the major possibilities. Not only does the sky blue in it's color contrast represent well, the sky, but also the Sylv part in it's name resembles an old west tale of a spirit of the wind, the element of the wind. Plus there are picture drawn by fans of Sylveon with the Unova Gym Leader Skyla based on their opinions. Plus the scarf on it's body and how they flow in the wind gives you a Flying type vibe of it. Also let's not forget the clues that point to it being a Bug type. Those exact clues also point to it being a flying type. Considering the butterfly bows and the other language names resembling that of a butterfly, but remember, not only is a butterfly a bug, but they have wing too, also making you believe it's a Flying type more.

Light: Not many people are sure that Pokemon will introduce the new type, but many people are wanting it to happen and if it were going to happen. Sylveon would in fact be that type considering what better way to introduce the type, but to make it interesting, introduce it in an Eeveelution. Though this possibility is as hard to determine as the rest unlike Flying.

Those are the main possibilities. But no one is for sure, I think that might make you feel better. Right?